An antenna is a device that transmits electrical signals into free space. The signals may be, for example, received by another antenna in a proximate or a distant location. A common antenna configuration is the well-known monopole antenna. A typical monopole consists of a straight wire mounted above and operating against a ground plane. A transmission arrangement such as a transmission line feeds electrical signals to the monopole with the ground plane serves as the ground potential for the transmission arrangement. An insulator is used to provide electrical separation between the monopole and the ground plane. As is well known in the art, the ground plane provides a mirror image for the monopole mounted above it so that from the perspective of the antenna it is as if another monopole antenna is located below the ground plane. In this way, the ground plane and the monopole antenna mimic a dipole antenna arrangement. For optimum performance of the monopole antenna at a particular frequency f of operation the length of the monopole antenna will be approximately one-quarter of the operating wavelength λ at that operating frequency f, or λ/4.
In general, for an antenna arrangement such as the typical monopole, the operating wavelength λ is related to the operating frequency f through the following relation:                     λ        =                  c                      f            ⁢                                          ɛ                r                                                                        (        1        )            where c is the speed of light in vacuum and εr is a relative permittivity associated with the insulator. Typically the operational frequency f is fixed by the application and the frequency limits design choices for the dimensional properties of the antenna.
Minimization of the space taken up by components is often of paramount importance in the design of devices such as wireless computing and other portable devices. For high-frequency applications that require antennas mounted on printed circuit boards, a typical monopole antenna arrangement may be impractical because of the antenna lengths at the high frequencies. A common substrate used to construct printed circuit boards is FR4® board has a relative permittivity εr of approximately 4.25. As an example of an antenna length at a high frequency, assuming that εr≅1, at an exemplary frequency of 5.25 GHz (5.25×109 Hz) the operating wavelength within the FR4 substrate will be approximately 57 millimeters (mm) and the corresponding λ/4 length of the antenna will be approximately 14 mm. For some applications, antennas with comparable lengths simply consume too much space in the vertical direction relative to the ground plane so as to be prohibitive in terms of their use.
The need to decrease the length of antenna configurations relative to a ground plane has led to a number of antenna arrangements, particularly in instances where horizontal space is available relative the ground plane. One example is the inverted L antenna arrangement. The inverted L is essentially a typical monopole antenna that is bent at approximately 90 degrees. Typically, the total length of the inverted L antenna, including the bent portion, will be λ/4, however a significant portion of that length may be in the bent portion that is approximately parallel to the ground plane. This decreases the length of the antenna portion that protrudes in the vertical direction relative to the ground plane. In most practical cases, this length will be no less than λ/8 due to the need to provide mechanical support for the bent portion of the antenna.
While this inverted L arrangement can achieve significant improvement in length reduction from the typical monopole antenna arrangement, better performance and length reduction can be achieved with the well-known top hat antenna. FIG. 1 is a diagram illustrating a side view of a traditional top hat antenna 100 mounted on a printed circuit board (PCB) 102. The top hat antenna 100 includes a disk or circular hat 104 of radius r and diameter d, and a cylindrical stem 106 of height h. Generally, the stem 106 and the circular hat 104 of the top hat antenna 100 are distinct pieces that are fused together via any of a series of well-known manufacturing processes to realize the top hat antenna 100. The top hat antenna 100 could also be machined from a single piece of metal. The PCB 102 includes a layer 108 of dielectric material, a ground plane 110, and a microstrip line or feed strip 112. The thicknesses of the dielectric layer 108, the ground plane 110, and the feed strip 112 are exaggerated relative to the top hat antenna 100 and to one another for purposes of illustration. For example, the feed strip 112 and the ground plane 110 are typically microthin layers of metal, for example, copper. The feed strip 112 includes a contact area 114 and forms a microstrip with the ground plane 110 and the dielectric layer 108 to provide electrical signals to the top hat antenna 100 at the contact area 114 where the strip 112 contacts the stem 106. Typically, the stem 106 of the top hat antenna 100 is soldered or otherwise fused to the feed strip 112 at the contact area 114. The dielectric layer 108 insulates the top hat antenna 100 from the ground plane 110. The top hat antenna 100 operates against the ground plane 108 to similarly mimic a dipole antenna effect.
The height h of the stem 106 together with the diameter d of the circular hat 104 are typically equal to one quarter of the operating wavelength λ at the operating frequency f, or λ/4. Typically, this implies that the height h of the stem 106 and thus the top hat antenna 100 approaches as low as λ/12. The top hat antenna 100 is an electrically small antenna, that is, the length of the antenna 100 is much smaller than the operating wavelength λ. In general, the performance of the traditional top hat antenna 100 at a particular operating frequency will vary according to the dimensions d and h of the antenna 100. Overall, the top hat antenna 100 provides substantial savings in terms of height relative to the ground plane 110.
One drawback of the traditional top hat antenna arrangement relates to mounting the top hat antenna on a PCB. The antenna is typically soldered or otherwise fused to the top of the PCB and to a microstrip line. Actually soldering the top hat antenna to the PCB is a complicated and mechanically precarious procedure in and of itself. The shape of the top hat antenna requires that an operator or a machine apply the solder at a difficult angle. A traditional monopole antenna does not present the same degree of difficulty in soldering. Soldering either the monopole or the top hat antenna to the top side of the PCB, however, is a process step that might not otherwise be necessary on the top side of the PCB but for the mounting of antennas. Put another way, a top hat antenna or a monopole antenna might be the only element that requires soldering to the top side of the PCB.
It would be desirable to provide a structurally stable arrangement for mounting an antenna that eliminates a soldering process on the top side of a printed circuit board, and that alleviates many of the difficulties inherent in mounting certain types of antennas on the printed circuit board.
An additional drawback of the traditional top hat antenna arrangement relates to manufacturability of the antenna. While a traditional top hat antenna may be machined from a single piece of metal, the antenna is generally formed by soldering, or by otherwise fusing, two distinct pieces of material to each other, one piece representing the circular hat, for example, and one piece representing the stem, for example. A manufacturing process that serves to accomplish this soldering or fusing together of pieces will typically be somewhat complicated and prone to error because of the lengths and the sizes of the pieces involved. As a result, the process typically proves to be fairly expensive on a per element basis and may be quite costly to implement on a mass production basis.
It would be desirable to provide an antenna of minimal length, in terms of its height when positioned above a ground plane, that is less complicated and less expensive to manufacture than a traditional top hat antenna but that does not significantly compromise performance relative to, for example, the traditional top hat antenna.